Singleton Pattern

Intent:
Ensure a class has only one instance and provide a global point of access to it.

When to Use:
When exactly one object is needed to coordinate actions across a system (e.g., logging, configuration, caching).

Key Characteristics:

Ensures a single instance
Provides a global access point
Thread-safe in multi-threaded environments (with locking mechanism)

Implementation Example (Python, Thread-safe):

# singleton.py

import threading

class Singleton:
    __instance = None
    __lock = threading.Lock()

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            with cls.__lock:  # Thread-safe instantiation
                if cls.__instance is None:
                    cls.__instance = super().__new__(cls)
        return cls.__instance

if __name__ == "__main__":
    instance1 = Singleton()
    print("Instance 1:", instance1)

    instance2 = Singleton()
    print("Instance 2:", instance2)

    print("Are both instances the same?", instance1 is instance2)
# singleton.py

import threading

class Singleton:
    __instance = None
    __lock = threading.Lock()

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            with cls.__lock:  # Thread-safe instantiation
                if cls.__instance is None:
                    cls.__instance = super().__new__(cls)
        return cls.__instance

if __name__ == "__main__":
    instance1 = Singleton()
    print("Instance 1:", instance1)

    instance2 = Singleton()
    print("Instance 2:", instance2)

    print("Are both instances the same?", instance1 is instance2)

Output:

Instance 1: <__main__.Singleton object at 0x...>
Instance 2: <__main__.Singleton object at 0x...>
Are both instances the same? True

Summary:

Guarantees a single instance
Useful for shared resources (loggers, config managers)
Thread safety is important in concurrent environments